gflags: add test (#28181)

Formula/gflags.rb

@@ -29,4 +29,34 @@ class Gflags < Formula
       system "make", "install"
     end
   end
+
+  test do
+    (testpath/"test.cpp").write <<~EOS
+      #include <iostream>
+      #include "gflags/gflags.h"
+
+      DEFINE_bool(verbose, false, "Display program name before message");
+      DEFINE_string(message, "Hello world!", "Message to print");
+
+      static bool IsNonEmptyMessage(const char *flagname, const std::string &value)
+      {
+        return value[0] != '\0';
+      }
+      DEFINE_validator(message, &IsNonEmptyMessage);
+
+      int main(int argc, char *argv[])
+      {
+        gflags::SetUsageMessage("some usage message");
+        gflags::SetVersionString("1.0.0");
+        gflags::ParseCommandLineFlags(&argc, &argv, true);
+        if (FLAGS_verbose) std::cout << gflags::ProgramInvocationShortName() << ": ";
+        std::cout << FLAGS_message;
+        gflags::ShutDownCommandLineFlags();
+        return 0;
+      }
+    EOS
+    system ENV.cxx, "-L#{lib}", "-lgflags", "test.cpp", "-o", "test"
+    assert_match "Hello world!", shell_output("./test")
+    assert_match "Foo bar!", shell_output("./test --message='Foo bar!'")
+  end
 end